The use of a stereoscopic image display on a monitor is, of course, advantageous since a feeling for the depth of the object being displayed is obtained which enhances the knowledge obtained by the viewer from the image displayed. Such displays, however, have typically been difficult to obtain.
Current digital stereoscopic display systems are based on individual selection devices which direct a different view of the same three dimensional object to the left eye than to the right eye. Using this technique, several commercial systems have been based on a time-multiplex technique in which left and right video images are presented sequentially. Observers view the alternate left and right images through electro-optical or mechanical shuttering devices synchronized to the alternation of left and right video images. These systems have been based upon standard commercial CRT monitors which refresh the screen at 60 Hz, non-interlaced.
A screen refreshed at the standard 60 Hz conventional time multiplexed rate, however, results in a perceived noticeable flicker to the eye of an observer because the number of screen refreshes per second is reduced from 60 to 30 per eye since two images are being viewed. An image that is perceived to be flicker-free is required to be refreshed at a rate of approximately 50 per second.
One attempt to overcome the flicker problem with stereoscopic imaging was to reduce the number of lines making up each of the left and right hand images by a factor of two. The two reduced resolution images were stored sequentially in a frame memory. A controller inserted an extra vertical sync pulse midway between each pair of vertical sync pulses in the sync output of the display controller. The modified sync signal was used to drive the monitor which had its vertical deflect circuitry modified to cover the active area of the screen in 1/120 sec. rather than 1/60 sec. The net effect was to alternate between half resolution right and left images at 120 Hz. Each time the display controller's frame memory was scanned, the screen was refreshed twice, once with the left image and once with the right. Flicker free imaging was, therefore, obtained with commercially available and conventional hardware. The division of a standard NTSC refresh field into two equal subfields results in the display of the left image on the even lines of the monitors viewing surface. The right image is displayed on the odd monitor lines. Since only half the resolution of the monitor is available for the display of each image, this result is less than satisfactory. Reference is made, in this regard, to the article entitled "A Flicker-Free Field-Sequential Stereoscopic Video System" by Lipton and Meyer, SMPTE Journal, Nov. 1984.